As you have found out, the usb 480Mbps is the burst speed in relationship to the cache. So it's alright for a few Mb and then will drop down to around 11Mbps.
The speed will drop down to 1Mbps or less after the cache is empty.
As Fruit Bat /\0/\ has mentioned the cpu has nothing to do with it, especially if DMA is enabled. this by-passes the cpu all together. If the device you are transfering to can support speeds over 15Mbps then your USB2.0 device should work a full speed.
e.g. a normal internal hard drive will run at 30Mbps +
An internal hard drive with a 5400 rotational speed and 2Mb cache would be around the 30Mb transfer rate. A 7200 internal hard drive with a 8Mb cache should be around 45Mb transfer speed. A SATA 150 drive with 8MB cache should be around 55Mb and a SATA 300 with 16Mb cache should be around 80MB.